Handling of USR2 signal in JVM on Linux

[Thomas Stüfe]

More specifically, I propose to gracefully ignore SIGUSR2 in release builds
if the receiving thread is not a java thread, but to retain the assert in
debug builds.

We could make it more involved by checking the sender pid and accepting
only signals from hotspot threads themselves, but I do not think this
complexity is necessary.

Cheers, Thomas

On Wed, Jun 15, 2022 at 12:26 PM Thomas Stüfe <thomas.stuefe at gmail.com>
wrote:

> SIGUSR2 is used by the hotspot, internally, to implement suspend/resume.
> It gets sent by hotspot via pthread_kill() to targeted threads to suspend
> them. In that case it is known that the receiving thread is a valid java
> thread and therefore the assert makes sense.
>
> However, as you describe SIGUSR2 can also be sent from outside via
> kill(2). In that case the receiving thread is arbitrarily chosen by the
> kernel. It is not necessarily a valid java thread. In that case the VM will
> crash (release) or assert (debug).
>
> I tend to think this is an error too. Or at least in grey area. Since this
> is very easy to fix in the hotspot, I'd suggest we do this.
>
> If nobody objects, I can file an issue and prepare the patch.
>
> Cheers, Thomas
>
> (P.s. not C++ undefined behavior, nothing to do with C++ :-)
>
> On Wed, Jun 15, 2022 at 12:11 PM Andrey Turbanov <turbanoff at gmail.com>
> wrote:
>
>> I mean, isn't JVM supposed to be safe? :)
>> Receiving this signal _could_ happen in a real deployment. And now, as
>> I can see, we have C++ undefined behaviour in release builds in this
>> case. Can we consider this as a bug?
>>
>> Andrey Turbanov
>>
>> вт, 14 июн. 2022 г. в 14:46, Alan Bateman <Alan.Bateman at oracle.com>:
>> >
>> > On 14/06/2022 10:44, Andrey Turbanov wrote:
>> > > Hello.
>> > > During investigation of signal handling in JVM (for
>> > > https://github.com/openjdk/jdk/pull/9100#discussion_r894992558 )
>> > > I found out that sending USR2 crashes my JDK. (Linux fastdebug x64)
>> > >
>> > > kill -USR2 1346792
>> > >
>> > > # assert(thread != __null) failed: Missing current thread in
>> SR_handler
>> > > # Internal Error
>> > >
>> (/home/turbanoff/Projects/official_jdk/src/hotspot/os/posix/signals_posix.cpp:1600),
>> > > pid=1346792, tid=1346792
>> > >
>> > > Full hs_err_pid1346792.log:
>> > > https://gist.github.com/turbanoff/2099327ea13357a90df43a2d6b0e2e6a
>> > >
>> > >
>> > > Is it known/expected behaviour?
>> > > I found some description there
>> > >
>> https://docs.oracle.com/en/java/javase/11/troubleshoot/handle-signals-and-exceptions.html
>> > > that USR2 is used for SUSPEND/RESUME. Is it supported by Hotspot?
>> >
>> > In general you have to be very careful when using signals. Yes, it can
>> > easily break things and probably notice it quickly with debug builds as
>> > asserts are compiled in to the builds (like the above). So I think
>> > you've found the right page to read up on this. In this case, you can
>> > set _JAVA_SR_SIGNUM to specify a different signal for S/R.
>> >
>> > -Alan
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/core-libs-dev/attachments/20220615/cc8c4212/attachment-0001.htm>